Abstract: The fracture reason of a friction welding joint for oil field drill rod was investigated by macro fracture analysis,SEM analysis,microstructure analysis,chemical composition analysis,XRD phase analysis and hardness test.The results show that the fracture surface was located at the connection weld between the rod head and rod body,which was a classical brittle fracture.There were a great number of ‘gray spot’ defects near the fracture surface of the rod head and rod body,which mainly were composed of Fe3O4.The defects were the main reason that reduced the combination ability of welding joint,which resulted in the brittle fracture of the drill rod joint during usage.